Kinematic mounts, otherwise known as kinematic couplings or restraints, are commonly used to couple measuring equipment or instruments to a base or substructure, where despite repeated disassembly and reassembly the plates remain in the same relative position to one another as when initially assembled. Examples of such instruments include: precision instruments, such as optical elements, including lenses mirrors, prisms, telescopes, cameras, lasers, sensors, or the like; sensitive measuring equipment; strain sensitive devices; lithography equipment, such as projection optics; and instruments that are disassembled and moved frequently so that a permanent support is not suitable.
Very small changes in the position of such instruments can make a substantial difference in the accuracy of results obtained from the instrument. Kinematic mounts were developed to address these small changes in the position during repeated assembly.
According to well-known principles, for a rigid body to be completely fixed in space, all six degrees of freedom need to be constrained. In other words, three translations and three rotations must be constrained with respect to some arbitrary fixed coordinate system. A mount is said to be kinematic when all six degrees of freedom are constrained without any additional constraints, i.e., any additional constraints would be redundant. A kinematic mount therefore has six independent constraints.
One well-known kinematic mount includes first and second plates. The first plate is generally fixed in space, while the second plate is free to move. The first plate has three V-shaped grooves formed therein, where each groove forms an angle of approximately 120 degrees with each other groove, and the walls of each groove form angles of approximately 45 degrees with the surface of the base plate. The second plate forms three depressions at the apexes of an equilateral triangle. The depressions are aligned with the grooves. During assembly, a spherical member is placed into each groove, contacting the two side walls of each respective groove at two point contacts. The second plate is then positioned onto the spherical members, such that each spherical member rests in a respective depression. In use, an instrument is be secured to the second plate. When the second plate is lifted from the first plate and replaced it will occupy the identical position relative to the first plate, which normally remains fixed.
However, the above described point contacts between each spherical member and groove leads to concentrated forces at these point contacts. These concentrated forces lead to high stresses, known as Hertzian stresses, both at the spherical member and at the groove.
Accordingly, the above described mount, while being sufficient for light loads, such as laboratory applications or light-duty field applications, fails in heavy-duty applications, such as when used in space launch vehicles, where high loads and high intensity vibrations and shocks cause failure at the point contacts.
Furthermore, when the kinematic mount is under a sufficiently heavy load, a depression or dent may be formed in the side walls of the groove that supports the spherical member. If the depression or dent is sufficiently deep, it may restrict the longitudinal movement of the spherical member in the groove. As a result, the second plate bearing the heavy load may not be accurately positioned, as accurate realignment typically requires some movement of the spherical members along the grooves.
In light of the above it is highly desirable to provide a kinematic mount that addresses the high stresses generated at the point contacts by heavy loads, while ensuring accurate realignment.